imo
trauma mechanically is slightly more “unique”, which is why its builds can feel more direct/specific opportune than just ex decay or poison, where decay builds ex seem largely non existent, and poison builds still tend to rely enough on flat(acid) to be less distinct. Trauma, granted some is also modifier related, completely ignoring armour means it has a different mitigation aspect that can play in your offensive selection, and ofc also defensive needs.
That’s got nothing really to be with “trauma” theme tho, and just quirk/collateral outcome of phys v armour mechanics or regular dot v resist and avg enemy phys res being that much lower, with certain skill modifier comboing allowing for high trauma to coast on by itself, or previous Octavious flat phys high trauma bonus warlords.
Vitality/decay i personally feel does nothing unique in GD, could be stripped now and in GD2 and nothing really would be lost besides a different red colour. I dont’ even really think it’s as thematically unique as chaos because it’s “everywhere” and unspecified (could be undead unique as example). And as i understood way back in early GD dev it was actually different and more close to TQ’s lifesteal/hp dmg usage? and got turned into regular dmg over time losing that inherent lifesteal dot effect and being more default bundled in? (for lack of better options?) - correct me if i’m misguided about GD og Vit dmg and lifesteal
phys/pierce/bleed still confusing the avg player might be an indicator it’s just not functioning that well, thematically or practically, trauma doesn’t really help or change much on that front besides making some people question what’s the different between taking a mace hit to the face and bleeding vs IT’ing, - and why don’t you always do both or sometimes only one or the other 
But i’ll again just take that as a quirk of GD engine/attempts of fixing TQ stuff (dots not working etc) and not deliberately intended to confuse players or be needlessly complicated for anyone but devs or experienced players.
Ironically i’m with the ones lamenting d2 immunities, and one might even point to d2r “fixing” it by introducing sunder to alleviate that gripe apparently enough had. But d2 did then have a more entertaining application of bleed/“trauma” with crushing blow/open wounds etc, and GD2 could do something similarly interesting making use of pierce/bleed/trauma without having them be just their own/separate generic dmg types like fire/poison etc.
Someone also recently brought up the difference between axes/maces(and swords?) i think. And while GD does apply a semi thematic approach with pierce/bleed/trauma there (unrelated to that weapon main speed scaling difference), it still doesn’t really change the actual function or application, or that conversion is equally the same regardless + dmg scaling (either can or can’t be converted etc bonus dmg sources are not restricted either)
i don’t think there being less physical dmg types than magick would be that significant, it’s already fewer, if we cut 3 magick types and 1½ phys type/interaction seems like it’s staying somewhat same split amounts
for me i think it should be something like
- tri ele gone entirely, or should atleast be something very unique
- Fire/burn can stay, cold and lightning can stay, too classic and ice shards are different from lightning strike anyway
- electro and frostburn i don’t think is necessary and could be cut or altered in effect, either more close to gd1 pseudo dot effect, or specifically tie them to status effects or debuffs like stun/freeze/shattering debuff effects and not so much a bonus dmg or dot dmg build type.
- Acid could go but could also stay; either way keep poison, this could either uniquely be the only dot or the dot focus build like how we might see bleed in GD1.
- Vit/decay gone, strip entirely as is or at minimum fully change mechanically to not be regular dmg but solely lifesteal/hp attacks etc. Either more like purely debuffing utility or highly unique build wise. - or do both while making it race specific, so ex only undeads have vit dmg/these new attacks
- Aether/Chaos, good colours, interesting world related, shouldn’t be restricted from player access/enemy race only, but perhaps make it more build/class thematic specialized, (- which would ofc also tie in with no applying the same conversion mechanic approach availability like GD1 ended up with).
- physical dmg keep,
- rethink and rework bleed/pierce/trauma effect and application as not just 3 other damages, but dont’ just simply merge pierce/bleed in GD2 (as otherwise been suggested for solution in GD1), apply one or the other more as effect and/or inherent related dmg+effect.
-bleed could become its own dmg/dot type build similar to poison, and “thematic immunities” could even still be a thing if so desired, if it was combined with “trauma”/crushing effects that could then work on ex skeletons (bones might not bleed but you can smack them to dust 
)
